This invention relates to threaded anchors used to reinforce a brittle wall member such as a plaster board when an article is to be fixed to the wall member by use of screws.
If nails or screws are directly driven into a brittle wall member such as a plaster board to fix an article to the wall member, they cannot engage the wall securely but come off easily. Thus, anchors or plugs for reinforcing the wall are needed.
Conventional such anchors have a hollow shank having a head at one end and an integral blade portion at the other end and formed with male threads on its outer periphery. Some blade portions are cylindrical with or without a serrated cutting edge at the tip, others are plate-shaped with a pointed tip, and still others are conical with an edge-defining recess on the outer periphery and adapted to bend when a mounting screw is driven in.
When an anchor with a cylindrical blade is driven into a wall member, chips tend to get into the blade and be compacted like a hard cork, which increases the resistance to driving the mounting screw, thus weakening the force with which the anchor is tightened in the wall member. When an anchor having a cylindrical blade with a serrated edge at its tip is brought into abutment with a wall member, the anchor tends to slip in the initial stage of boring due to a small contact area with the wall member. It is thus difficult to drive in such an anchor.
In the case of an anchor with a flat plate-shaped or conical blade, chips of the wall member are discharged outwardly, thus messing the finish when the anchor has been driven in. Also, the wall tends to collapse at its back due to weak chip discharging force.
Moreover, when the mounting screw is driven in, especially if the screw is a long one, the blade portion has to be inclined and the bottom of the hollow shank of the anchor has to be broken. This weakens the force with which the anchor is tightened in the wall member.
Furthermore, since conventional anchors of all types have inner diameters corresponding to individual mounting screws, each anchor can be used with only limited kinds of mounting screws. Conventional anchors also have a problem in that they tend to come off together with a mounting screw when the latter is unscrewed.
An object of the present invention is to provide a threaded anchor which provides a beautiful finished surface when set, increased tightening force, and which can be used with increased variety of mounting screws, and an which never turns together with the mounting screw when the screw is turned in a loosening direction.